In the context of a cellular telephone network, an unauthenticated mobile emergency call is an emergency call where the subscriber is not fully authenticated or registered. A call to a Public Safety Answering Point (PSAP), i.e. an emergency centre, where the subscriber's number is not visible, may also be considered an unauthenticated mobile emergency call. Such unauthenticated calls have been permitted in order to allow an emergency call to be made despite normal telephone services being unavailable to a particular user terminal. Examples of unauthenticated call cases include:                A call is placed before the user has logged into the network, e.g. a user makes an emergency call prior to entering a PIN code (to unlock a phone or SIM lock) by dialing 112 on a terminal. In this case, the user may or may not be the owner of the phone.        A user has a valid subscription but his operator does not have a roaming agreement with the operator the user is camping on. An example can be when the user is in an area where only his operator's competitor has coverage, or when a user is travelling abroad.        The user has had his subscription barred for normal calls. This could be the case, for example, when the user has not paid his bills or is out of cash on his pre-paid card.        A user terminal does not have a Universal Integrated Circuit Card (UICC) with credentials for authentication (e.g. SIM or USIM application). This is the case in the clear majority of unauthenticated emergency calls.        
Taking Sweden as an example, the number of unauthenticated emergency calls in the year 2000 was of the order of 700000, contributing around 18% of all emergency calls from the mobile networks. Out of these 700000 unauthenticated calls, only 1.5% were made in respect of real emergencies. The possibility of making unauthenticated calls encourages hoax calls, as the callers can effectively remain anonymous. In some countries including the UK, network operators have inhibited unauthenticated calls on mobile networks in an attempt to reduce the number of hoax calls. Of course, this in turn prevents users from making valid emergency calls on those rare occasions when only an unauthenticated terminal is available.
Current terminal and network implementations allow the terminal to omit the user identity (that is the International Mobile Subscriber Identity (IMSI) in the case of GSM/UMTS) in the emergency call setup if the credentials are missing. [NB. The IMSI is only available to the terminal if the UICC is present and unlocked.] Instead, the terminal will include an equipment identifier (such as the International Mobile Equipment Identifier (IMEI) number in the case of GSM/UMTS) in the call setup request. FIG. 1 shows a signalling flow associated with an emergency call where the SIM/USIM is assumed to be present in the user terminal and unlocked. If no SIM/USIM is present and unlocked, the authentication will fail, but the emergency call may continue. The IMEI will be included in the CM Service Request in place of the IMSI (or TMSI).
In the case of IP Multimedia Subsystem (IMS) [which is a 3GPP initiative to provide rich, multimedia services over 3G and other networks and utilises Session Initiation Protocol (SIP) signalling between client based SIP User Agents and IMS network nodes] unauthenticated emergency calls are described in 3GPP TS 23.167 and 3GPP TS 24.229. In short, the UE creates an anonymous emergency request, including the equipment identifier, and sends this to the P-CSCF within the IMS core. The P-CSCF checks that it is allowed to make an unauthenticated emergency call and, if so, forwards the request to the emergency CSCF (E-CSCF). Using a Location Retrieval Function (LRF), the E-CSCF finds the closest PSAP to route the call to. As with conventional Circuit Switched (CS) calls, users have the possibility to make anonymous hoax calls.